The overall goal of this proposed research is to assign a role in hepatocarcinogenesis to each of several genetic changes commonly identified in human and/or mouse liver tumors. To accomplish this objective, a newly developed, novel in vivo assay system is employed, the comparative hepatocyte growth assay. As starting material, this assay uses genetically modified mice generated by standard transgenic or gene-targeting methodologies. Hepatocytes are isolated from these mice, and then transplanted into liver of specially designed recipient mice. Evaluation of the subsequent clonal growth of donor cells in the new host environment allows quantification of the influence of single or combined genetic modifications on (1) the rate at which hepatocytes proliferate under growth stimulatory conditions; (2) the capacity for sustained hepatocyte growth in a quiescent liver environment; and (3) the risk for hepatocyte progression to malignant transformation. Each measure reflects an important characteristic of neoplastic cells. The quantitative nature of the data provides a more refined understanding of the specific contribution of each genetic change, alone and in combination with others, to the process of liver cancer progression. The final objective is to use this system as a tool for cancer gene discovery, by correlating changes in patterns of gene expression with the changes in hepatocyte clonal growth that will be identified. This proposal has the following specific aims. Aim 1: Define cancer incidence, latency, multiplicity, histotype, and progression in mice with defined combinations of hepatocyte gene changes. Aim 2: Quantify growth alterations and transformation frequency associated with defined hepatocyte gene changes in vivo. Aim 3: Identify biological behaviors and molecular characteristics of transformed hepatocytes. Successful completion of these experiments will (1) define specific interactions between several highly relevant genetic changes (synergistic or additive complementation, or no complementation) during hepatic carcinogenesis; (2) provide a systematic and comparative evaluation of the influence of genetic and environmental alterations, selected for their relevance to liver cancer, on hepatocyte growth potential; and (3) will identify gene sets that cooperate with oncogenes during tumor progression.